ARM has published a new open-source X.Org DDX Linux graphics driver while working to enable support for their next-generation ARM Mali T6xx graphics core.
Tom Cooksey explained in a mailing list message this morning that the Mali T6xx approach is nearly identical to how the Texas Instruments OMAP graphics are handled. They're using a kernel DRM driver to manage the display controller via kernel mode-setting, the KMS driver handles the basic buffers with GEM support, etc. The client also uses the new PRIME ioctls for exporting the GEM buffer.
For the ARM Mali T6xx Linux enablement, they are only using a DRM driver for driving the display controller while they have their own separate kernel driver for poking and handling the GPU itself. ARM though isn't being too open-source friendly in terms of a fully open stack or providing proper documentation.